As the capacities of the storage apparatuses that use magnetic recording media become larger, the flying heights of the magnetic heads have been lowered to improve the recording densities. The magnetic recording medium must have a very flat and smooth surface, that is to have a very precise surface structure, to lower the flying height of a magnetic head. For example, it is required that the conventional nonmagnetic metal substrate, such as an aluminum (Al) substrate, be machined precisely.
The conventional nonmagnetic metal substrate and the conventional magnetic recording medium using the conventional nonmagnetic metal substrate are manufactured in the following way.
Usually, a blank disk, prepared by rolling a molten metal, heating the rolled metal, annealing the heated metal and machining the annealed metal to have the predetermined dimensions, is used for a nonmagnetic substrate. The blank disk is machined to have a predetermined inner diameter and a predetermined outer diameter. The flatness and the smoothness of the machined blank disk is improved by lapping. A Ni—P layer of 13 μm in thickness is plated on the blank disk to improve the surface hardness thereof. The surface of the Ni—P layer is polished to a surface roughness Ra of 10 Å. Final lapping using diamond slurry is applied to the polished surface of the blank disk. Laser zone textures are formed in the contact start stop (CSS) zone of the thus obtained substrate such that the bump height is, for example, 190 Å and the bump density is, for example, 30×30. Finally, the substrate is washed meticulously, resulting in a substrate for magnetic recording media.
A chromium (Cr) undercoating layer of 500 Å in thickness, a Co-14Cr-4Ta magnetic layer of 300 Å in thickness and a carbon protection layer of 80 Å in thickness are deposited one after another on the substrate for magnetic recording media by the DC sputtering method. The surface of the as deposited laminate is burnished using a burnishing tape and a fluorine lubricant layer of 20 Å in thickness is formed on the burnished surface by dip-coating or by spin-coating, resulting in a magnetic recording medium.
The method of manufacturing the conventional substrate for magnetic recording media and the method of manufacturing the conventional magnetic recording medium are becoming more complicated to meet the recent requirements for a higher recording density. Moreover, it is required to manufacture a magnetic recording medium with reduced costs while maintaining high functions. Novel magnetic recording media, that use a plastic substrate, have been proposed to meet these contradictory requirements.
A method that manufactures a plastic substrate by molding and, at the same time, forms the CSS zone thereof with excellent productivity, is advantageous to industrially provide magnetic recording media with low manufacturing costs.
However, the plastic substrates manufactured by injection molding synthetic resin pellets are inferior to the metal substrates and the ceramic substrates, such as a glass substrate, from the view points of surface flatness and smoothness, since rugged surface portions of several μm in height difference and micro waviness are caused by molding. The rugged portions and the micro waviness are hazardous for realizing a very flat and smooth surface, which is a prerequisite for the magnetic recording medium.
The magnetic recording medium formed on the substrate including the rugged portions and the micro waviness is also hazardous for reading out data to the magnetic head and for writing data from the magnetic head. Especially when a low-flying-height head conducts continuous seek at a high speed, the flight thereof is not stabilized and, in the end, head crash is the result. Thus, the conventional plastic substrate impairs the durability of the magnetic recording medium.
Polycarbonate resins and poly(methyl methacrylate) resins are used for the material of the plastic substrate for magnetic recording media. In addition to these resins, thermoplastic norbornene resins are used for the material of the plastic substrate for magnetic recording media. The thermoplastic norbornene resin, that exhibits excellent thermal resistance, low hygroscopicity and excellent shape stability, is useful to provide excellent magnetic recording media. However, the problems described above are posed also on the substrate made of the thermoplastic norbornene resin.
The commercial products of the thermoplastic norbornene resin include APEL supplied from Mitsui Chemicals, Inc. and ZEONEX supplied from Nippon Zeon Co., Ltd.